Retaining ring for holding semiconductor wafers in a chemical mechanical polishing apparatus

ABSTRACT

A retaining ring to be fitted on a chemical mechanical polishing apparatus for semiconductor wafers is disclosed, the retaining ring being of integral design made of a plastic material, wherein the retaining ring forms on a first front side thereof a bearing surface for supporting the retaining ring on a polishing surface of the polishing apparatus, and includes on the side thereof lying opposite the first front side thereof in axial direction fitting elements for fitting the retaining ring on the polishing apparatus.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This patent application is a continuation-in-part application ofcopending U.S. patent application Ser. No. 10,322,428, filed Dec. 19,2002, which is incorporated by reference, and which claims the benefitof German Patent Application No. 102 47 180.0, filed Oct. 2, 2002, whichis incorporated by reference.

FIELD OF THE INVENTION

[0002] The invention relates to a retaining ring for holdingsemiconductor wafers in a chemical-mechanical polishing apparatus.

BACKGROUND OF THE INVENTION

[0003] Nowadays, integrated circuits are typically formed onsemiconductor substrates, particularly silicon wafers, by the sequentialdeposition of conductive, semiconductive and insulative layers on thewafer. After deposition of each layer, etching is performed to createthe circuitry functions. After a series of layers have been sequentiallydeposited and etched, the uppermost surface of the semiconductorsubstrate, i.e., the outer surface of the substrate, becomesincreasingly non-planar. This non-planar surface presents problems inthe photolithographic steps of the integrated circuit fabricationprocess. Therefore, there is a need to periodically planarize or leveloff the semiconductor substrate surface.

[0004] So-called chemical mechanical polishing (CMP) is one of theaccepted methods for this. This planarization method typically requiresthat the substrate, i.e., the semiconductor wafer, be mounted on acarrier or polishing head. The exposed surface of the substrate is thenpressed against a rotating polishing pad. A controlled force is exertedon the substrate via the carrier head to press the substrate against thepolishing pad. A polishing agent containing at least one chemicallyreactive substance and abrasive particles is supplied to the surface ofthe polishing pad.

[0005] A recurring problem in the CMP process is the so-called edgeeffect, i.e., the tendency to polish the edge of the substrate at adifferent rate than the center of the substrate. This typically resultsin over-polishing at the edge, i.e., the removal of too much materialfrom the edge, particularly at the outermost 5 to 10 mm of a wafer of200 mm in diameter.

[0006] Over-polishing reduces the overall flatness of the substrate andmakes the edge of the substrate unsuitable for integrated circuitfabrication and therefore decreases the process yield.

[0007] To solve this problem, U.S. Pat. No. 6,251,215 discloses aretaining ring be made of two portions; a first portion being made of arigid material, namely a metal portion, and a second portion ofa-plastic material, which is less rigid, so that, on the one hand, itcan be subjected to abrasion, and, on the other hand, it will not damagethe semiconductor wafer when contacting it.

[0008] Owing to the edge conditions that prevail in chemical mechanicalpolishing, U.S. Pat. No. 6,251,215 discloses that the plastic portion ofthe retaining ring and the metal ring are bonded to one another with anepoxy adhesive. Alternatively, it is disclosed that the two portions arejoined together with a press fit.

[0009] In practice, both solutions prove to be inadequate.

[0010] While the plastic portion is held securely on the metal portionwhen the two portions are bonded with epoxy adhesive, the reconditioningof the retaining ring after the plastic portion has been subjected to acertain amount of abrasion presents problems. The current practice is tosend the complete retaining rings to the manufacturer where the plasticportion is mechanically removed and the metal portion is then heated upto approximately 200° C. to thermally decompose the adhesive residuesthereon. Subsequently, the metal portion has to be sandblasted in orderto remove final residues of the adhesive, and only then can a newplastic ring be adhesively attached thereto.

[0011] Owing to this time-consuming and costly procedure, the retainingrings as such become very expensive. In addition, the metal portions,which are more expensive to produce than the plastic portions, onlywithstand a small number of cycles of reconditioning, in particular, onaccount of the temperature treatment for thermal decomposition of theadhesive and the sandblasting treatment that is subsequently required.

[0012] Exchanging a used plastic ring when metal and plastic portionsare joined with a press fit is easier, but a press fit for joining theplastic and metal portions has proven unsuitable for reliablywithstanding the forces that occur during the polishing process.

[0013] The present invention, relating to a retaining ring that can bemanufactured more cost-effectively and, in particular, fitted morecost-effectively with a new plastic part, provides for ameliorating atleast some of the disadvantages of the prior art. These and otheradvantages of the present invention will be apparent from thedescription as set forth below.

BRIEF SUMMARY OF THE INVENTION

[0014] In an embodiment, the invention provides a retaining ring,wherein the retaining ring is of integral design and is made of aplastic material, and the retaining ring forms on a first front sidethereof a bearing surface for supporting the retaining ring on apolishing surface of the polishing apparatus, and includes on the sidethereof lying opposite the first front side thereof in axial directionfitting elements for fitting the retaining ring on the polishingapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIGS. 1A to 1D show a first embodiment of a retaining ringaccording to the invention;

[0016]FIGS. 2A to 2D show a second embodiment of a retaining ringaccording to the invention;.

[0017]FIG. 3 shows an enlargement of a detail from a sectionalrepresentation of a third embodiment of a retaining ring according tothe invention;

[0018]FIGS. 4A and 4B show a fourth embodiment of a retaining ringaccording to the invention; and

[0019]FIGS. 5A and 5B show a fifth embodiment of a retaining ringaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In accordance with an embodiment of the present invention, aretaining ring is provided, the retaining ring being of integral designand. made of a plastic material, the retaining ring forming on a firstfront side thereof a bearing surface for supporting the retaining ringon a polishing surface of the polishing apparatus, and including on theside thereof lying opposite the first front side thereof in axialdirection fitting elements for fitting the retaining ring on thepolishing apparatus.

[0021] The inventive construction of the retaining ring permitscost-effective manufacture thereof, and the retaining ring can,therefore, be disposed of in its entirety after excessive wear.

[0022] In order to impart sufficient stability to the retaining ring,which ensures that the retaining ring will not undergo deformation inits geometry during the polishing process and thereby cause unevenremoval of material from the semiconductor wafer, the plastic materialis selected on the basis of its mechanical properties.

[0023] The retaining ring is preferably made up like a sandwich of atleast two layers or components.

[0024] The plastic material may, for example, include a thermoplasticmaterial, a thermosetting plastic material, an elastomer and/or aplastic composition.

[0025] A considerably larger range of plastic materials is availablewhen the plastic material is used in reinforced, in particular infiber-reinforced, form.

[0026] It is then desirable for the plastic material to exhibit adjacentto its first front side, i.e., the side forming the bearing surface forsupporting the retaining ring on a polishing surface of the polishingapparatus, no reinforcement materials or a lower content ofreinforcement materials than on its side including the fitting elements.

[0027] This results in a partial area of increased mechanical stability,which stabilizes the retaining ring overall in its geometry, in the areaheld on the polishing apparatus. On the side on which the abrasive wearoccurs, the retaining ring then exhibits lower stability and, inparticular, is less hard, so that contact between the semiconductorwafers to be treated and the plastic material is more gentle on thewafers.

[0028] In order to reduce the wear and optimize the tribologicalproperties, abrasion-reducing and/or wear-reducing additives, forexample, PTFE, polyimide, molybdenum disulfide, graphite, boron nitride,nanoparticles or the like, may be added to the plastic material.

[0029] Finally, it is also conceivable for the reinforcement materialsto be limited to a core area of the retaining ring, in particular, thearea in which the fitting elements are then also arranged. This impartsthe necessary rigidity and geometrical integrity to the retaining ring,also when one-sided loads occur, while contact between the retainingring and the wafer will not cause any damage to the wafer as the waferonly comes into contact with the softer plastic materials.

[0030] A simple but cost effective method for providing such preferredstructure having an area of increased mechanical stability in suchportion of the retaining ring where it is held on the polishingapparatus whereas another area contacting the polishing surface of thepolishing apparatus is less hard resides in manufacturing in one step afirst retaining ring portion of a plastic material of high mechanicalstability, especially a reinforced resin material, while in another stepa second ring portion is manufactured of a plastic material which hasbeen optimized for contacting the polishing surface of the polishingapparatus.

[0031] The resin material for the second ring portion plastic materialmay be the same as the resin material which forms the basis for thefirst ring portion plastic material.

[0032] The resin material for the second portion may be selectedindependently from the resin material for the first ring portion,however, use of the same resin material is preferred.

[0033] The resin material of the second ring portion may be modified inorder to reduce wear and to optimize tribological properties as hereinbefore described in connection with another embodiment of the invention.

[0034] The first and second ring portions can be attached to one anotherby a friction welding process to form a unitary structure.

[0035] As an alternative the first and second ring portions may beinjection moulded from two different components (plastic materials) inone step to form the unitary structure.

[0036] An alternative solution is for the retaining ring to comprise ametal ring embedded in the plastic material and arranged concentricallyin the retaining ring. The metal ring then provides the retaining ringwith the necessary rigidity and geometrical integrity, while thesurrounding plastic material ensures that the semiconductor wafer to betreated will not be subjected to any mechanical damage.

[0037] The fitting elements will then preferably be held on the metalring, so that the fitting elements, which, of course, serve to fit theretaining ring on the polishing apparatus, ensure that the retainingring Will lie exactly on the polishing apparatus, and therebyadditionally maintain the geometrical structure of the retaining ring.

[0038] The metal ring, which is surrounded by the plastic material, mayremain uncovered, in particular, on the upper side, i.e., on the side ofthe retaining ring facing the polishing apparatus, as there is noprovision for contact with the wafer material here. Also, this side ofthe retaining ring is scarcely or not at all exposed to the chemicalagents of the chemical mechanical polishing process.

[0039] It is, however, preferable for the metal ring to be completelysheathed by the plastic material, as a larger range of metallicmaterials is then available for manufacture of the metal ring becausethe metallic material is completely protected by the plastic materialagainst chemical attack by the agents used in the chemical mechanicalpolishing.

[0040] The simplest form of metal ring for reinforcing the retainingring is a sheet metal ring. This may, in particular, be perforated orgenerally provided with through-holes, so that a positive connection isestablished between the plastic material and the metal ring by theplastic material passing through the through-holes.

[0041] As the rigidity of the retaining ring, in particular, in axialdirection, is of great importance, a sheet metal ring with asubstantially cylindrical shape, i.e., in the form of a cylinder wall,will preferably be used.

[0042] Alternatively, ring-shaped disks made of metal may also be used,and a larger thickness of the sheet metal material may then be required.

[0043] The metal ring of the afore-discussed embodiments of theinvention may be pretreated on at least a portion of its surface priorto embedding it into the plastic material. Such pretreatment serves toenhance bonding of the plastic material to the metal of the metal ring.

[0044] Suitable pretreatments are chemical etching, plasma etching, sandblasting, and metal spraying.

[0045] Metal spraying is the most preferred pretreatment. Using thistechnology a coating of metal beads may be formed on the surfaceportions of the metal ring to be pretreated. The coating of metal beadscan be applied as a so-called mono-layer, however, superimposed multiplelayers are preferred.

[0046] Very good results are obtained when a coating of a thickness of700 μm or more is applied, the preferred average diameter of the metalbeads being 300 μm to 600 μm, more preferably 400 μm to 500 μm.

[0047] The thickness of the coating may be 1 mm or more, however,thicknesses of much more than 1 mm do not show a further increase inbonding strength.

[0048] The metal beads coating produced by the metal plasma sprayingprocess provides for numerous microscopic recesses and interstices intowhich the plastic material flows when the metal ring is embedded in theplastic material, especially when using an injection moulding process.

[0049] Preferably the plastic material is made to enter the recesses andvoids volumes at least partly so as to establish an interlocking bondingbetween the plastic material and the metal beads coating.

[0050] While the metal ring is preferably made of steel, the metal beadsare preferably prepared from copper or a copper alloy.

[0051] Each of the components of the invention will now be described inmore detail below, wherein like components have like reference numbers.

[0052]FIGS. 1A to 1D show an embodiment of a retaining ring 10 accordingto the invention, which is made of a fiber-reinforced plastic materialand has a substantially rectangular cross section. Adjacent the outercircumference 12, the retaining ring 10 has an axially projectingcircumferential collar 14 in which threaded bushings 16 are incorporatedat regular angular intervals. The threaded bushings 16 serve to attachthe retaining ring to the chemical mechanical polishing apparatus.

[0053] The side of the retaining ring 10 opposite the collar 14 forms abearing surface 18 with which the retaining ring 10 rests on a polishingsurface of the polishing apparatus when the polishing apparatus is inoperation.

[0054] Owing to the retaining ring 10 being cost-effectively made ofplastic, when the bearing surface 18 is worn the retaining ring 10 as awhole can be disposed of easily, and the problem of reusing parts of theretaining ring 10 is dispensed with.

[0055]FIG. 1B shows the retaining ring 10 according to an embodiment ofthe invention taken along line A-A in FIG. 1A. FIG. 1C shows anenlargement of a detail taken from the sectional representation of FIG.1B. Finally, FIG. 1D again shows a perspective illustration of theretaining ring 10 according to the invention.

[0056] The circumferential collar 14 has on its surface pointing towardsthe polishing apparatus an opening 20 for engagement with acomplementary projection on the polishing apparatus in the fitted state,so as to facilitate correct angular orientation of the retaining ring 10in relation to the polishing apparatus and enable automatic alignment ofthe threaded bushings 16 with corresponding through-holes on the,polishing apparatus, through which screw bolts extend.

[0057] Further embodiments in the form of a retaining ring 30 are shownin FIGS. 2A to 2D and FIG. 3. The design of the retaining ring 30differs from the design of the retaining ring 10, in particular, in thata metal ring 32 is embedded in the plastic material as reinforcingelement for the plastic material. The metal ring is arrangedconcentrically with the retaining ring 30 and is made of a perforatedsheet material. Therefore, when injecting the plastic material aroundthe metal ring 32, the plastic material will pass through thethrough-holes of the perforated material and thereby anchor the metalring 32 with a positive connection in the retaining ring 30. In FIG. 3,the metal ring 32 remains uncovered on the upper side, i.e., on the sideof the retaining ring 30 facing the polishing apparatus.

[0058] The retaining ring 30 also has on its side facing the polishingapparatus a collar 34 arranged adjacent to the outer circumferentialsurface 36 of the retaining ring 30 and projecting in axial direction.Threaded bushings 40 for mounting the retaining ring 30 on the polishingapparatus are arranged at regular angular intervals in the area of thecollar 34.

[0059] To facilitate alignment of the threaded bushings 40 withcorresponding screw bolts on the polishing apparatus, the collar 34 hasan opening 42 for engagement with a projection on the polishingapparatus, thereby to ensure correct angular assembly of the retainingring 30.

[0060] Owing to the use of a metal ring 32 which is essentiallycompletely surrounded by the plastic material, the plastic material maybe selected from a wider spectrum and, in particular, optimally adaptedto the requirements for resistance to the abrasion from the polishingsurface of the polishing apparatus.

[0061] The rigidity of the retaining ring 30 is essentially guaranteedby the stability and the geometrical integrity of the metal ring 32(annular disk). The metal ring 32 may simultaneously serve to carry thethreaded bushings 40, and the metal ring 32 with the threaded bushings40 can then be placed in a prefabricated state in an injection moldingtool and embedded in plastic by injection molding. Owing to the factthat the metal ring 32 is essentially completely surrounded by theplastic material of the retaining ring 30, the metallic material fromwhich the metal ring 32 is made may also be selected from a wide rangeof materials, as there is scarcely any or no contact at all between themetal ring and the chemical agents used for the chemical mechanicalpolishing of the semiconductor wafers. There is therefore no need toanticipate corrosion problems.

[0062]FIGS. 4A and 4B show another embodiment of the invention where aretaining ring 50 is composed of a first ring portion 52 which is madeof a first plastic material, a second ring portion 54 made of a secondplastic material and a number of threaded bushings 56 incorporated atregular angular intervals in the first ring portion 52.

[0063] The first ring portion 52 and second ring portion 54 are attachedto one another via a bonding layer 58. The faces of the first and secondring portions 52 and 54 which meet at the bonding layer 58 areessentially identical in their radial extensions.

[0064] The bonding layer 58 is created when friction welding the firstand second ring portions together to form a unitary structure.Alternatively, the bonding layer 58 is formed when the first and secondring portions are simultaneously injection moulded in a common mould andthe first and second plastic materials are contacting one another.

[0065] While the first plastic material is selected such that itprovides high stability to the retaining ring, the second plasticmaterial is selected to optimize wear and tribological properties. Thefirst plastic material is preferably a reinforced resin material whilethe second plastic material incorporates preferably no or a lower amountof reinforcing components. The second plastic material preferablycontains wear reducing components since it is designed to contact thepolishing surface of the polishing apparatus.

[0066] The resin material used in the first and second plastic materialmay be the same or different forming a matrix to hold the reinforcingand wear reducing components, respectively. Preferably, the resinmaterial for the first and second plastic material is the same.

[0067]FIGS. 5A and 5B show a further embodiment of a retaining ring 60of the invention. The retaining ring 60 comprises a metal ring 62 whichis embedded in a plastics material 64. The metal ring 62 is preferablymade of steel. On a surface of the metal ring 62 contacting the plasticsmaterial 64 a coating 66 of metal beads, preferably of copper or acopper alloy, is deposited which provides for numerous micro-intersticeswhich allow the plastic material to flow into the coating 66 therebyproviding a positive connection between the metal ring 62 and theplastic material 64 upon the injection moulding process.

[0068] The surface portion of the metal ring 62 bearing the coating 66is preferably limited by a rim 68 the height of which may beapproximately 1 mm. The metal beads forming the coating 66 preferablyhave an average diameter of 400 μm to 500 μm. The metal beads may bedeposited on the surface portion of the Metal ring 62 as a mono-layer,however, a multiple layer structure as schematically indicated in FIG.5B is preferred. For creating a firm bond or positive lock between themetal ring 62 and the plastics material 64 it is not necessary that theplastics material 64 fills all of the voids volumes of the intersticesbetween the metal beads.

[0069] All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and, specificallyindicated to be incorporated by reference and were set forth in itsentirety herein.

[0070] The use of the terms “a” and “an” and “the” and similar referentsin the context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

[0071] Preferred embodiments of this invention are described herein,including the best mode known to the inventors for carrying out theinvention. Of course, variations of those preferred embodiments willbecome apparent to those of ordinary skill in the art upon reading theforegoing description. The inventors expect skilled artisans to employsuch variations as appropriate, and the inventors intend for theinvention to be practiced otherwise than as specifically describedherein. Accordingly, this invention includes all modifications andequivalents of the subject matter recited in the claims appended heretoas permitted by applicable law. Moreover, any combination of theabove-described elements in all possible variations thereof isencompassed by the invention unless otherwise indicated herein orotherwise clearly contradicted by context.

What is claimed is:
 1. A retaining ring for a chemical mechanicalpolishing apparatus for semiconductor wafers, comprising: a retainingring of integral design made of a plastic material, wherein theretaining ring forms on a first front side thereof a bearing surface forsupporting the retaining ring on a polishing surface of the polishingapparatus, and includes on the side thereof lying opposite the firstfront side thereof in axial direction fitting elements for fitting theretaining ring on the polishing apparatus.
 2. The retaining ringaccording to claim 1, wherein the retaining ring comprises at least twolayers or components.
 3. The retaining ring according to claim 1,wherein the plastic material comprises at least one of a thermoplasticmaterial, a thermosetting plastic material, and an elastomer.
 4. Theretaining ring according to claim 1, wherein the plastic material is areinforced plastic material.
 5. The retaining ring according to claim 4,wherein the plastic material is a fiber-reinforced plastic material. 6.The retaining ring according to claim 4, wherein the plastic materialhas a lower content of reinforcement substances adjacent to its firstfront side than on its side including the fitting elements.
 7. Theretaining ring according to claim 1, wherein abrasion-reducing and/orwear-reducing additives are admixed with the plastic material.
 8. Theretaining ring according to claim 1, wherein the retaining ringcomprises a metal ring embedded in the plastic material and arrangedconcentrically in the retaining ring.
 9. The retaining ring according toclaim 8, wherein the fitting elements are held on the metal ring. 10.The retaining ring according to claim 8, wherein the metal ring iscompletely encased by the plastic material.
 11. The retaining ringaccording to claim 8, wherein the metal ring is a sheet metal ring. 12.The retaining ring according to claim 11, wherein the sheet metal ringis a perforated sheet metal ring.
 13. The retaining ring according toclaim 11, wherein the sheet metal ring has a substantially cylindricalshape.
 14. The retaining ring according to claim 12, wherein the sheetmetal ring has the shape of an annular disk.
 15. The retaining ringaccording to claim 2, wherein the plastic material comprises at leastone of a thermoplastic material, a thermosetting plastic material, andan elastomer.
 16. The retaining ring according to claim 2, wherein theplastic material is a reinforced plastic material.
 17. The retainingring according to claim 16, wherein the plastic material is afiber-reinforced plastic material.
 18. The retaining ring according toclaim 17, wherein the plastic material has a lower content ofreinforcement substances adjacent to its first front side than on itsside including the fitting elements.
 19. The retaining ring according toclaim 3, wherein the plastic material is a reinforced plastic material.20. The retaining ring according to claim 19, wherein the plasticmaterial is a fiber-reinforced plastic material.